1. Field of the Invention
The present invention relates generally to the field of image registration and more specifically to the field of image registration configured for medical applications.
2. Related Art
Deformable image registration is a technique for modifying a deformable image in an elastic way to match similar features in a reference image. The technique, in general, involves determining a transformation necessary to register (e.g., to align) two images by matching structures of interest in the deformable image with structures of interest within the reference image. The matching process includes deforming the deformable image to achieve alignment of these structures within the two images. The deformable and reference images may each be images of a patient at different times. The deformable image may be an image of a healthy person and the reference image may be an image of a patient, or vice versa.
For example, the deformable image may include an image of a patient in which a physician has carefully delineated a treatment volume, and the reference image may include an image of the same patient recorded at a later time during treatment. A treatment volume is a spatial volume within a patient to be treated using radiation such as X-rays or particle beams. By deforming the carefully delineated image to align with an image recorded during treatment, movement of the treatment volume within the patient can be readily identified. The unaltered reference image, having a treatment volume identified through the image registration process, can then be used to target the new position of the treatment volume during treatment.
Various deformation algorithms exist in the prior art. One of these deformation algorithms is referred to as the demons algorithm. One advantage of the demons algorithm is the speed at which the steps of the algorithm are performed (e.g., on the order of minutes).
In some situations, the images are taken using different techniques. For example, one image may be generated using computed (axial) tomography (CT) technology and an other image may be generated using Cone Beam CT (CBCT) technology. CBCT images sometimes include low frequency artifacts (e.g., false features) that are the result of the cone beam CT image generation process. One disadvantage of the existing registration techniques is that they do not account for these low frequency artifacts. For example, an artifact can cause misregistration when attempting to match CT and CBCT images using the demons algorithm. There is, therefore, a need for improved systems and methods of registering medical images.